Heat exchangers in the related art include those adopting a four-pass structure that includes a plurality of tubes disposed over two rows to the front and the rear along the direction of airflow through which the coolant is caused to flow in the top-bottom direction, an upper tank portion communicating with the upper ends of the tubes and a lower tank portion communicating with the lower ends of the tubes (see Patent Reference Literature 1).
A tendency whereby the coolant flowing through an upper tank portion 100 flows in greater quantities to the tubes present on the upstream side along the coolant flowing direction due to gravity and the coolant flowing through a lower tank portion 101 flows in greater quantities to the tubes present on the downstream side along the coolant flowing direction due to the inertial force, as shown in FIG. 5(a) is often observed in a heat exchanger adopting the four-pass structure described above. This tendency leads to a lowered coolant flow rate over an area A at a first pass portion 110, an area B at a second past portion 111, an area C at a third pass portion 112 and an area D at a fourth pass portion 113 which, in turn, allows the temperature over these areas to rise readily. In particular, the temperature change over an area E (see FIG. 5(b)) formed with the part of the area A at the first pass portion 110 and the part of the area D at the fourth pass portion 113 overlapping each other along the front/rear direction of the airflow causes a disruption in the temperature distribution in the entire heat exchanging unit. The tendency becomes more pronounced when the coolant is circulated at a low flow rate.
The problem discussed above is addressed in the evaporator disclosed in Patent Reference Literature 1 by forming a plurality of restriction holes at the second pass portion and the fourth pass portion on the lower tank portion side so as to adjust the coolant flow rate (see Patent Reference Literature 1).
Patent Reference Literature 1: Japanese Unexamined Patent Publication No. 2001-74388